1. Field of the Invention
The present invention relates to a lens for light collection for total reflection for a front monitor photodiode (PD) in an optical pick-up device using a holographic unit.
2. Description of the Related Art
Recently, optical disc techniques have been highly developed. Also, demand for an inexpensive CD-R/RW(Compact Disc-Recordable/Re-Writable), which is an optical disc recording device, has greatly increased.
Although it is essential to provide a stable laser power in optical disc recording, there may be a fluctuation in laser power due to optical feedback of a light beam from an optical disc.
In order to solve this problem, a drive current adjusting method may be used, in which laser power is monitored to ensure that the laser power is constant. For this laser power monitoring, a monitor photodiode (PD) may be used.
Typically, a monitor PD is arranged in rear of a laser diode in order to conveniently monitor the laser power of the laser diode. However, such a rear monitor PD has a drawback in that it exhibits a degraded accuracy because it utilizes a rearwardly-emitted optical output from a laser diode which is more non-linear than a forwardly-emitted optical output from the laser diode.
In order to solve this problem, a front monitor PD may be used which directly monitors a forwardly-emitted optical light beam.
FIG. 1 illustrates the configuration of a general optical system used for a front monitor PD in a CD-R/RW using a high-power holographic pick-up module as an optical pick-up.
A light beam outputted from a holographic pick-up module (HPM) 101 is converted into a collimated beam while passing through a collimator lens 102. The collimated beam is transmitted through a beam splitter 103, and then reflected by a mirror 104. The reflected light beam is converged by an objective lens 105, so that it is focused onto an optical disc 106. A zero-order one of the light beam outputted from the HPM 101 is partially incident to a monitor PD 107 by the beam splitter 103 so that it is utilized in the adjustment of recording optical power.
However, the above mentioned conventional optical system used in the front monitor PD has a drawback in that it exhibits a degraded light using efficiency because it utilizes a part of the zero-order beam outputted from the HPM.
Furthermore, this optical system degrades the optical characteristics of the monitor PD. In addition, there is a drawback in that the axial distance of the monitor PD from a light source is too great.